Chemical Engineering July 2022 - 44

Consideration
TABLE 1: COMPARISON OF CONVEYOR TECHNOLOGIES WITH " STANDARD " FEATURES*
Screw
Pneumatic
(dilute
phase)
Materials conveyed
Capacity
Distance
Vertical lift
Layout flexibility, horizontalturns/corners
possible?
Cohesive, sticky materials
Highly abrasive materials
Dust containment
Toxic materials
Particle degradation
Cleanable/sanitary
Maintenance requirements
High temperatures
Ability to use inert gas
Cross-contam./cleanout
Multiple inlets possible?
Multiple outlets possible?
Steady/continuous feed
Upfront capital cost
Specific energy cost
Most
Low
(< 30 ton/h)
Long
(<1,000 ft)
Excellent
Excellent
Good
Very poor to
poor
Excellent
Excellent
Very poor
Excellent
Very low
Excellent
Excellent
Excellent
Excellent
Excellent
Excellent
Low
Very high
Pneumatic
(dense
phase)
Limited
Moderate
(<300 ton/h)
Very long
(<2,000 ft)
Excellent
Excellent
Very poor
Moderate
Excellent
Excellent
Fair to good
Excellent
Low
Excellent
Excellent
Excellent
Poor to good
Excellent
Poor
Moderate
Moderate
generally have much lower energy
costs, which reduces opex. On the
other hand, mechanical conveyors
also have more moving parts and
are typically more maintenanceintensive
that pneumatic systems.
If the mechanical conveyor is not
properly selected for your application,
frequent maintenance requirements
could significantly increase
the conveyor's opex, so that the
mechanical conveyor could ultimately
cost more in the long run.
Additionally, some lower-capex mechanical
conveyors may be more
prone to erosion, breakdown or
failure versus other lower-capex
mechanical conveyors, so a lowercapex
conveyor may actually have
much higher opex and may cost you
more over the life of the equipment
compared to a scenario where you
44
Belt
Most/all
Extremely high
(<5,000 ton/h)
Extremely long
(near unlimited)
Poor
Very poor
Excellent
Excellent
Most
High
(<600 ton/h)
Moderate
(<100 ft)
Poor
Poor
Moderate to
good
Moderate to
good
Fair, if enclosed Very good
Very poor
Good
Poor
Moderate
Very poor
Avoid
Good
Excellent
Poor
Excellent
Moderate to
high
Very low
Very good
Poor to fair
Poor to
moderate
Moderate
Excellent
Good
Very poor
Excellent
Good, with
gates
Moderate
Moderate to
high
Low
*Specialized conveyors in each category may have unique features and expanded capabilities
spent a little more up-front. It's all a
balancing act.
So, now with those questions
and considerations in mind, how
do you move ahead in selecting a
conveyor? The next sections of the
article explore the features, advantages
and disadvantages of some of
the most common conveyor types.
Note that " standard " features are
discussed for each category. There
are many specialized features and
other conditionals that could improve
each conveyor's characteristics
and capabilities.
Pneumatic conveyors
Pneumatic conveying involves using
air or another gas to transport the
solid material through a pipeline, at
moderate to high velocities. Pneumatic
systems are often highly attractive,
since they generally have low
capex, and they are also by far the
most flexible conveying technology
from a layout standpoint. Pneumatic
systems can easily support multiple
pickup and delivery points and it is
fairly simple to route piping through
a plant. Also, pneumatic systems
do not occupy a lot of space, and it
is easy to run both horizontally and
vertically. They can often use existing
pipe racks or supports, and can be
run over long distances.
There are two main modes of
pneumatic transport:
* Dilute-phase - Particles are fully
suspended in the conveying gas, at
relatively low pressures drops, high
velocities, and low solids loadings
(typically 3-15 psig and 50-80 ft/s
or more at pickup).
* Dense-phase - Particles are only
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Bucket
elevator
Limited
Very high (std.
<1,000-2,000
ton/h)
Long
(<300 ft)
Very good
Very poor
Moderate
(avoid digger
designs)
Good (avoid
digger designs)
Moderate
Moderate
Poor
Very
poor
High
Excellent
Good
Poor-Good
Very poor, if
digger design
Very poor
Poor
Moderate
Low
Tubular
drag (cable/
chain)
Limited
Low
(<100 ton/h)
Moderate
(<300 ft)
Good
Good
Very poor
Very poor to
poor
Excellent
Very good
Excellent with
cable, Poor to
fair with chain
Good to
excellent
Moderate to
High
Fair
Excellent
Good
Excellent
Excellent
Moderate
Moderate
Low
Most
High
(<1,000 ton/h)
Long
(<500 ft)
Moderate
Very poor
Moderate
Good
Good
Moderate
Poor
Very poor
High
Excellent
Good
Very poor
Excellent
Good, with
gates
Good
Moderate to
high
Low
Most
Low
(<100 ton/h)
Short
(<50 ft std.)
Very poor
Limited
Fair to good
Excellent
Poor to
excellent
Good, if enclosed
Excellent
Excellent
Very
low
Excellent
Good
Very good
Very poor
Very poor
Excellent
Moderate
Very low
Drag chain
Vibratory
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Chemical Engineering July 2022

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